Heat exchangers for cooling catalyst particles in the form of fine grade material by indirect contact with a cooling fluid are well known. Heat exchangers of this type maintain the catalyst in a fluidized state with a fluidizing medium that passes upwardly through the catalyst. A series of conduits comprising tubes, channels or coils are positioned within the catalyst bed. A fluid passes through the conduits to remove heat from the catalyst by indirect heat exchange. Catalyst is continuously supplied to the fluidized bed and fluidized catalyst is continuously withdrawn from the bed. Methods of supplying or withdrawing catalyst from the bed through an exchanger include flow-through and backmix type exchangers. There are two basic versions of flow-through coolers; one uses gravity feed wherein catalyst enters an upper inlet and exits a lower outlet, and the other employs fluidized transport that moves catalyst from a lower inlet past the cooling conduits and out an upper outlet. In a backmix operation, catalyst is circulated through a common inlet and outlet that exchanges catalyst between the cooler and the source of the hot catalyst particles.
Indirect heat exchangers of the above described type have been finding increasing use on the regenerators of processes for the fluidized catalytic conversion of hydrocarbons. The FCC process has been extensively relied upon for the conversion of hydrocarbon streams such as vacuum gas oils and other relatively heavy oils into lighter and more valuable products. In the FCC process, starting hydrocarbon material contacts a finely divided particulate catalyst which is fluidized by a gas or vapor. As the particulate material catalyzes the cracking reaction, a by-product of the cracking reaction referred to as coke is surfacedeposited thereon. A regenerator, which is an integral part of the FCC process, continuously removes coke from the catalyst surface by oxidation. Oxidation of the coke releases a large amount of heat which in part supplies the heat input needed for the cracking reaction. As FCC units have been called upon to process heavier feeds, greater amounts of coke must be removed in the regeneration zone with a corresponding increase in the amount of heat generated therein. This additional heat poses a number of problems for the FCC process. The excess heat can upset the thermal balance of the process thereby requiring a lowering of the circulation of hot catalyst from the regenerator to the reactor which in turn can lower the yield of valuable products. In addition, the excess heat may raise temperatures to the point of damaging the equipment or catalyst particles. Therefore, it is advantageous to have a means of lowering the regenerator temperature. For reasons of temperature control and process flexibility, heat exchangers having cooling tubes located outside the regenerator vessel have become the method of choice.
An important consideration in the FCC process is the transport of the catalyst. It is often difficult to incorporate a heat exchanger having the necessary dimensions to provide the desired degree of heat transfer into the constraints of the process arrangement. In the main, these constraints involve obtaining sufficient exchanger length to accommodate the required surface area of the exchanger conduits and providing inlet and outlets for the movement of the catalyst between the exchanger and the rest of the process unit. For some FCC process units, addition of a particle heat exchanger may necessitate raising the entire structure, or the incorporation of extra conduits and fluidization devices in order to meet the exchanger design requirements. When the particle heat exchanger is added to a newly designed FCC unit, the increased elevation and/or added conduits and fluidization devices increase costs and complicate construction of the unit. It is also popular to retrofit catalyst heat exchangers into existing FCC process units. In these cases, the structural constraints may not only add to the cost of the unit, but may not permit the incorporation of a catalyst exchanger having the desired heat removal capacity.
The use of a backmix type exchanger, as previously mentioned, will simplify the incorporation of the catalyst heat exchanger into the FCC unit since it only requires the use of a single inlet/outlet conduit. However, the overall heat exchange capacity of this type of device is limited by the amount of catalyst circulation that can be obtained over its vertical length. Moreover, the overall heat transfer per length of cooling conduit available in the backmix cooler is lower than in the flow-through type exchanger where catalyst flows from an inlet in one end of the heat exchanger to an outlet at the opposite end. Finally, an additional layout constraint of the backmix type cooler is its need for a very large inlet/outlet conduit in order to obtain adequate particle circulation between the heat exchanger and a retention space where the particles are withdrawn and retrieved. Therefore, backmix type exchangers cannot overcome many of the layout problems associated with the incorporation of a remote catalyst heat exchanger into an FCC process unit.